We had a driver in our center (woman) that farmed in addition to driving. She scheduled her vacations around when she hoped to plant, harvest, cut and bale hay (she also fed cow/calf pairs.)
Her husband helped a bit and her son did also. I am pretty sure that she needed to hire help with haying, but was able to plant and harvest on her own.
Before UPS, I milked cows (I was a herdsman at a family farm.) When I quit that, I row cropped with my brother in law and ran a small feedlot (we finished dairy steers in batches of 100.)
There is no way that I would do a UPS gig AND farm. There would be way too many days of coming home and working past midnight and then working all weekend. If I stayed on the farm, I would rent the land and buildings.